This invention relates to a detector circuit having an improved noise cut-off characteristic.
An AM radio receiver or a transceiver may receive extremely small input signals. In this case, an AGC circuit enhances the power gain of the circuit at the stage preceding the intermediate-frequency amplifier circuit. As a result, the noise generated by the high-frequency amplifier circuit is much amplified, and the electro-acoustic converter circuit at the final stage generates noise quite annoying to the ears.
With reference to FIG. 1 the drawbacks of a conventional detector circuit 10 will be explained more in detail.
As shown in FIG. 1, the intermediate-frequency amplifier circuit 1 has its input terminal coupled to the output terminal of the high-frequency amplifier circuit (not shown). The output terminal of the intermediate-frequency amplifier circuit 1 is coupled to the input terminal 11 of the conventional detector circuit 10, i.e. the base of an NPN transistor TR1. In the NPN transistor TR1, the PN junction between the base and emitter functions as a detector element. The collector of the transistor TR1 is connected to a power source V.sub.CC, while the emitter is grounded through a resisor R1 and connected to a low-frequency output terminal 12. To the resistor R1, a capacitor C1 is connected in parallel. The base of the transistor TR1 is applied with a DC bias voltage E.
FIG. 2 is a V-I characteristic curve which shows the relationship between the emitter current I of the transistor TR1 and the voltage V between the input terminal 11 (i.e. the base of the transistor TR1) and output terminal 12 of the detector circuit 10. The current at a bias point A is (E-V.sub.BE)/R1, where "V.sub.BE " denotes the base-emitter voltage of the transistor TR1. If voltage V.sub.B between the input and output terminals 11 and 12 which corresponds to the bias point A is made the reference voltage of high-frequency input signals, even input signals of minute amplitudes are inevitably rectified or detected since the V-I characteristic curve rises very sharply. In this case, the noise in the input signals of minute amplitudes is detected, too, and is inevitably supplied to the electro-acoustic converter circuit in the form of a noise signal.